1. Field of the Invention
The present invention relates to a method of controlling battery charging and discharging in a hybrid car power source, and in particular, to a method that controls charging and discharging to keep remaining battery capacity within a targeted range. Further, the present invention relates to a method of controlling charging and discharging that narrows the targeted range of control for safe charging and discharging when an abnormality is detected.
2. Description of the Related Art
In a hybrid car power source, battery charging and discharging is controlled to keep remaining battery capacity within a targeted range. For example, the targeted control range for remaining battery capacity is set to 50%±20%, which is a range from 30% to 70%. By keeping the remaining battery capacity within the targeted range, over-charging and over-discharging can be prevented and battery life can be extended. Controlling remaining battery capacity within a targeted range having a center of 50% maintains a state where the batteries can always supply power to the motor and where the batteries can be charged by regenerative braking. Therefore, the batteries can be used effectively to drive the hybrid car and achieve high mileage. By widening the targeted control range of the batteries, the effective battery capacity that can actually be used is increased. This is because the actual discharge capacity is increased and the actual charging capacity for regenerative braking is also increased. Consequently, to increase battery capacity as a practical matter, it is necessary to widen the targeted control range.
However, widening the targeted control range increases the probability of over-charging or over-discharging, which causes battery degradation. Further, it is a challenging task to consistently measure remaining capacity without error for batteries being charged and discharged. For example, remaining battery capacity can be determined by integrating battery charging and discharging current, but if the current sensor malfunctions and current cannot be measured accurately, remaining battery capacity is then determined from battery voltage and temperature. In this case, since remaining battery capacity is determined from battery voltage and temperature, accurate determination may not be possible. As a result, even when charging and discharging are controlled to keep the detected remaining battery capacity within the targeted range, it may not be possible to keep the actual remaining battery capacity within the targeted control range. In addition, if battery conditions become abnormal, internal pressure can rise and a safety valve can open to discharge gas or electrolyte solution. Even in this type of abnormal situation, the probability of being able to drive the hybrid car using the batteries can be increased by providing a mode of operation that allows battery charging and discharging, as well as a mode that prohibits any charging or discharging. The hybrid car can use the batteries most effectively with a mode that can drive normally with both the batteries and the engine, a mode that can drive with the batteries and the engine and while keeping the batteries within a narrower targeted control range, and a mode that prohibits any battery charging or discharging and can only drive with the engine. However, since various detrimental effects can occur when an abnormality occurs and the targeted control range is wide, a method to narrow the targeted control range during an abnormality has been developed. (Refer to Japanese Patent Application Disclosure 2002-354684.)